SMN2 and proximal spinal muscular atrophy: Age-dependent differences in BBB permeability, neuropil density, and cell tropisms may contribute (Foust et al., 2009, 2010); however, the early temporal requirement for SMN has been corroborated using mice with inducible SMN alleles (Le et al., 2011; Lutz et al., 2011; Kariya et al., 2014a), and is consistent with human and mouse SMN being most highly expressed in the CNS perinatally (Jablonka and Sendtner, 2017), and the most common and severe form of SMA (type I) manifesting before 6 months of age.